1. Field of the Invention
This invention relates to heat exchangers and catalytic reactors used in aircraft and propulsion systems. In particular, the invention relates to heat exchangers which convert liquid fuel to gaseous fuel and reactors which change the chemical composition to gaseous fuel.
2. Description of Related Art
Future aircraft and propulsion systems will require very high levels of cooling by the fuel used to power the aircraft. Fuels contemplated for use with the present invention include cryogenic fuels, such as liquid methane or liquid propane, more exotic noncryogenic hydrocarbon fuels such as methylcyclohexane, and the more conventional aircraft hydrocarbon fuel mixtures such as JP-4, JP-5 or JET-A. It is well known to use heat exchangers to cool aircraft and aircraft engine parts using the sink capacity of fuel. It also known to use a reactor having a heat exchanger with more exotic non-cryogenic hydrocarbon fuels such as methylcyclohexane.
The present invention also provides for using the fuel's additional heat sink capacity obtained from the latent heat and the sensible heat of the fuel (due to the fuel changing from liquid to gas) in addition to an increase in temperature. Furthermore, for certain fuels the present invention makes it more feasible to achieve additional heat sink by changing the chemical composition of the fuel.
Numerous applications for heat exchangers and reactors are envisioned for future aircraft and propulsion systems. These applications include structural cooling needed for supersonic and hypersonic aircraft, cooling of air used to cool turbine blades in high performance turbojet engines and cooling for special applications.
It is generally well known to construct heat exchangers and reactors for this type of application from tubes which contain the fuel. Hot air is typically directed over the tubes which, at elevated temperatures, cause the fuel to undergo auto-oxidation reactions with oxygen dissolved in the fuel to produce deposits generally in the form of polymer gums and sediments which adhere to the tube wall. At very high temperature, coke may be produced as the result of both polymerization and pyrolysis; i.e. molecular fragmentation. Regardless of the many complex chemical factors, the tubular heat exchanger can tolerate only a very limited amount of deposit formation on the tube wall since these deposits have very low thermal conduitivity and simply serve to insulate the tube from the fuel. This drastically reduces the efficiency of the heat exchanger and poses a potential risk of engine failure as well as increased costs of operation and construction.
It is generally known that improvements can be made by removing oxygen, maintaining tight control of fuel chemistry, and avoiding contact with adverse catalytic agents such as certain pure metals. However, such methods are not economically feasible nor simple, even with the use of ordinary liquid JET-A kerosene and its economical conversion to gaseous fuel.
Very high heat sink fuels rely on chemical conversion in addition to sensible heating and change in phase. One such fuel is methylcyclohexane (MCH). The endothermic reaction is: EQU Methylcyclohexane+Heat.fwdarw.Toluene+Hydrogen EQU C7H14.fwdarw.C7H8+3H2
For applications which relate to this invention it has been common practice to consider a tubular reactor in which the fuel flows through a catalytic pack bed inside the tubes. The packed bed is typically fine beads of aluminum oxide interspersed with a metal such as platinum. This design approach is basically the same as described previously for the heat exchanger. The heat exchange and reactor functions are greatly diminished by fuel deposits formed on the tube wall that form and serve as an insulator. Furthermore, any deposits formed on the packed bed poisons the bed and diminishes the capability for chemical conversion related to the reactor function.
Recognizing that the catalytic bed is an auxiliary function to heat transfer across the tube wall, it is evident that pressure drop associated with the bed is a negative effect and in fact is a major drawback to this common design approach. The catalyst bed results in enormous fuel pressure drop, not otherwise evident in the same open tube as used for the plain heat exchanger.
In conventional designed heat exchangers and reactors, the tubes containing fuel are surrounded by hot air. Also, the ends of the tubes are usually brazed to tube headers. This results in numerous possibilities for leakage of high pressure fuel to lower pressure air. For applications envisioned by the present invention, limits of flammability in terms of temperature, pressure and fuel/air composition indicate possibilities for either fire or explosion. Neither possibility is acceptable in a manned aircraft.
Heat pipes are well known thermal devices for the efficient transport of thermal energy. A heat pipe is a closed structure containing a working fluid that transports thermal energy from one part, called the evaporator, where heat is supplied to the device, to another part, called the condenser, where heat is extracted from the device. Heat is transferred by means of liquid vaporization in the evaporator, vapor flow in the core region, vapor condensation in the condenser, and condensate return to the evaporator by capillary action in the wick.